Motor vehicles are, for safety reasons, increasingly being fitted with monitoring systems comprising sensors mounted on each of the wheel-tire assemblies of the vehicle, these being devoted to measuring parameters, such as tire temperature or pressure, and intended to inform the driver of any abnormal variation in the parameter being measured.
These monitoring systems usually comprise a sensor positioned in each wheel-tire assembly with a microprocessor and a radiofrequency transmitter, and a central processing unit that receives the signals emitted by the transmitters, with a processor incorporating a radiofrequency receiver connected to an antenna.
One of the problems that such monitoring systems have to overcome lies in the need to associate, with each signal received by the receiver of the central processing unit, information as to the location of the sensor and therefore as to which wheel-tire assembly has sent this signal, this need persisting throughout the life of the vehicle, that is to say still having to be met even after wheel-tire assemblies have been changed or, more simply, have been swapped around.
Document EP 1 669 221 A1 discloses a self-contained device intended to be carried by a wheel-tire assembly in order to locate whether the assembly is positioned on the right or on the left of a vehicle, and comprising:                two magnetic sensors with axes of maximum sensitivity;        means of measuring a signal at the terminals of each of the magnetic sensors, these means being capable of delivering two periodic signals which are phase shifted relative to one another and each representative of the variations in the magnitudes of the magnetic field as detected by the magnetic sensors during a revolution of the wheel-tire assembly; and        a processing unit programmed to determine, from the phase shift between the two periodic signals, the direction in which the wheel-tire assembly is rotating and to deduce, from this direction of rotation and from the direction of travel of the vehicle, whether the wheel-tire assembly is located on the right or on the left.        
This device is such that the axes of maximum sensitivity of the two magnetic sensors are intended to be positioned in the wheel-tire assembly at a plane secant to the axis of rotation of the wheel-tire assembly, and offset from one another, in this secant plane, by a predetermined angle.
The two sensors in this device detect the overall magnetic field surrounding them, which is made up of the Earth's magnetic field combined with the vehicle's environmental magnetic field. This vehicle environmental magnetic field is the resultant of a collection of vehicle fields, which are created by the presence of electrical or magnetic equipment on board the vehicle and by metallic components close to the wheel arches such as the brake calipers and the components of the ground-contact system.
There are, however, certain points on the globe and certain directions of vehicle travel for which the contribution of the Earth's magnetic field as measured by the two sensors of the above device becomes negligible in comparison with the contribution of the environmental magnetic field, and this may present problems in exploiting the signals.